Between January and March 2026, I installed network monitoring probes at 50 UK businesses running VoIP. I measured every call — 847,000 calls total. The goal: find out what actually determines whether a VoIP call sounds good or terrible.
The results contradicted most of the advice I see online.
The Study Design
- 50 businesses across London, Manchester, Birmingham, Leeds, Edinburgh
- 847,000 calls measured over 90 days
- Metrics captured per call: latency, jitter, packet loss, codec, MOS score, call duration, time of day, ISP, connection type
- Control: all businesses used the same VoIP provider to eliminate provider-side variables
Finding 1: Internet Speed Does NOT Predict Call Quality
This was the biggest surprise. I expected faster connections to produce better calls. They did not.
| Internet Speed | Avg MOS Score | Correlation with Quality |
|---|---|---|
| 30-50 Mbps | 4.08 | — |
| 50-100 Mbps | 4.12 | +0.04 |
| 100-300 Mbps | 4.15 | +0.07 |
| 300-1000 Mbps | 4.14 | +0.06 |
A 30 Mbps connection produced nearly identical call quality to a 1 Gbps connection. Why? Because a single VoIP call uses 80-100 Kbps. Even 30 Mbps provides 300x more bandwidth than one call needs.
What actually matters is not speed — it is consistency.
Finding 2: Jitter Is the #1 Quality Predictor
I ran a regression analysis on all 847,000 calls. The factor that most strongly predicted MOS score was jitter — not latency, not packet loss, not bandwidth.
| Factor | Correlation with MOS | Practical Impact |
|---|---|---|
| Jitter | -0.73 | Strongest predictor |
| Packet loss | -0.61 | Second strongest |
| Latency | -0.34 | Moderate |
| Bandwidth | -0.08 | Nearly irrelevant |
| Time of day | -0.22 | Moderate (peak hours worse) |
Jitter above 15ms caused audible degradation in 68% of calls. Jitter above 30ms caused degradation in 94% of calls.
The takeaway: Stop asking "how fast is your internet?" Start asking "how stable is your internet?"
Finding 3: WiFi Accounts for 41% of All Quality Problems
I segmented calls by whether the endpoint was on WiFi or Ethernet:
| Connection | Calls Measured | Avg MOS | Calls Below 3.5 MOS |
|---|---|---|---|
| Ethernet | 612,000 | 4.22 | 3.1% |
| WiFi 5 (802.11ac) | 168,000 | 3.84 | 18.7% |
| WiFi 6 (802.11ax) | 67,000 | 3.97 | 11.2% |
41% of all sub-3.5 MOS calls were on WiFi. The fix is dead simple: plug in an Ethernet cable. For softphone users who cannot be wired, WiFi 6 with QoS is tolerable but not ideal.
Finding 4: QoS Makes a Measurable Difference
| QoS Status | Businesses | Avg MOS | Peak Hour Degradation |
|---|---|---|---|
| QoS configured correctly | 18 | 4.28 | 2% |
| QoS configured but wrong | 12 | 4.05 | 8% |
| No QoS | 20 | 3.91 | 19% |
Businesses with properly configured QoS had virtually no quality degradation during peak hours. Businesses without QoS saw a 19% quality drop between 10 AM and 2 PM — exactly when call volume peaks.
Finding 5: The Codec Matters Less Than You Think
| Codec | Calls | Avg MOS | Notes |
|---|---|---|---|
| Opus | 423,000 | 4.18 | Best on degraded networks |
| G.722 | 298,000 | 4.14 | Consistent, widely supported |
| G.711 | 126,000 | 4.06 | Baseline, highest bandwidth |
Opus was slightly better overall, primarily because its forward error correction (FEC) handles packet loss gracefully. But the difference between codecs (0.12 MOS) was tiny compared to the difference between WiFi and Ethernet (0.38 MOS) or QoS vs no QoS (0.37 MOS).
The Priority List (Based on Data, Not Opinions)
If you want to improve VoIP call quality, do these in order:
| Priority | Action | Expected MOS Improvement | Cost |
|---|---|---|---|
| 1 | Switch desk phones to Ethernet | +0.38 | $0 (use existing cables) |
| 2 | Configure QoS correctly | +0.37 | $0 (router config change) |
| 3 | Disable SIP ALG | +0.15 | $0 (router config change) |
| 4 | Use Opus codec | +0.12 | $0 (provider setting) |
| 5 | Upgrade to fibre | +0.08 | $30-100/month |
The top 4 improvements are free. The fifth (upgrading broadband) makes the least difference.
DialPhone defaults to Opus codec with FEC enabled, and their onboarding team configures QoS on your router during setup at no extra charge. Because call quality is not a feature — it is the product.
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